RACEMIC ACID AND MESOTARTARIC ACID 133
The yield varies with the period of boiling, but usually does not
exceed 10 grams.
Resolution of Racemic Acid,—The racemic acid is dis-
solved in water (250 c.c.) and divided into two equal volumes.
Half of the solution is carefully neutralised with caustic soda
and the other half with ammonia, and the two solutions then
The liquid is concentrated and poured into a crystallising dish.
If, on cooling, the crystals are small and massed together, the
solution has been too concentrated, and must be diluted so that
small, well-defined crystals deposit. A dozen or so of these are
picked out, dried, and put on one side. The remaining crystals
are re-dissolved and left to cool in a room of fairly even tempera-
When the solution is just cold the crystals, previously re-
moved, are sown evenly over the bottom of the dish at distances
of i—2 cms. apart and left for two days. The crystals will have
now grown to a size which will enable the facets to be readily
recognised. Each crystal is dried and carefully examined with
a pocket lens in order to determine the position of the hemi-
hedral facets, and placed in separate heaps. These facets lie
to the right or left hand of the central prism face, as shown in
Fig. 74. The crystals should be weighed, dissolved, and the
solution diluted and examined in the polarimeter. The specific
rotation may then be calculated. See Appendix, p. 264.